1. Field of the Invention
The present invention relates to a development roller for use in an image formation apparatus such as an electrophotographic copying machine, a printer and a facsimile apparatus.
2. Discussion of Background
In recent years, in the field of electrophotographic image formation apparatus such as copying machines, printers and facsimile apparatus, in particular, in the field of small size electrophotographic image formation apparatus, the maintenance thereof is being significantly simplified. An electrophotographic image formation apparatus, using a dry type mono-component toner, is used in practice as a low cost apparatus due to its small size and easy maintenance.
In such an electrophotographic image formation apparatus, a latent electrostatic image is formed on a surface of a photoconductor drum serving as a latent-electrostatic-image bearing member, and a charged toner is supplied onto the surface of the photoconductor drum, with a concentration of the toner being maintained constant, by a development roller which serves as a toner carrying member. With the toner being brought into contact with the photoconductor drum or being placed near the photoconductor drum, the latent electrostatic image formed on the surface of the photoconductor drum is developed with the toner to a visible toner image. The thus developed visible toner image is then transferred to an appropriate image transfer sheet such as a recording paper, and then fixed thereto, for instance, with the application of heat and pressure thereto, in an image fixing process. Thus, the toner image is recorded on the image transfer sheet and then output.
As such a development roller, various types of development rollers have been proposed, for example, those including an elastic layer composed of an elastic material such as a rubber or an elastomer. In order to prevent the photoconductor from being contaminated by a plasticizer and a low-molecular-weight component released from a rubber or urethane foam used in the elastic layer due to the bleeding thereof from the elastic layer, and also in order to prevent the tacking or filming of toner, the elastic layer is generally covered with a surface layer made of a resin with excellent toner releasability which is capable of preventing the bleeding of the plasticizer and the low-molecular-weight component from the elastic layer onto the surface of the photoconductor.
For example, in Japanese Laid-Open Patent Application 53-3233, there is proposed a method of developing a latent electrostatic image to a visible image, in which an elastic latent-electrostatic-image-bearing member which bears thereon a latent electrostatic image and an elastic development roller are brought into pressure contact with each other at a development position, and the latent electrostatic image on the latent-electrostatic-image-bearing member is developed to a visible image while the pressure between the latent-electrostatic-image-bearing member and the elastic development roller is being absorbed by the elasticity of each of the latent-electrostatic-image-bearing member and the elastic development roller. In this latent electrostatic image development method, in order to bring the development roller into contact with a photoconductor drum, the development roller is composed of a shaft made of a metal and a conducting rubber with about 40HS which is caused to adhere to a peripheral surface of the shaft.
In this latent electrostatic image development method, when the development roller which bears thereon a thin layer of toner is brought into contact with a latent electrostatic image formed on the photoconductor drum, the thin layer of toner is transferred from the surface of the development roller to the surface of the photoconductor in accordance with an electric field for development formed between the photoconductor drum and the development roller, so that the latent electrostatic image formed on the photoconductor drum is developed to a visible toner image. This latent electrostatic image development method has an advantage over other development methods that a colored image can be easily formed by using a mono-component non-magnetic toner, without using any magnetic material in the toner.
Furthermore, a dielectric layer containing an electric resistivity adjustment agent such as carbon is generally provided on the surface of such an elastic development roller. For example, in Japanese Laid-Open Patent Application 10-115979, there are disclosed a method of decreasing the abrasion of a surface of a development roller, and a method of adjusting an electrostatic capacity or an electric resistivity of the development roller, by coating a surface of a conducting elastic layer with a resin component which is different from a resin component used in the conducting elastic layer. As a resin component for use in a surface layer provided on the conducting elastic layer, a resin which does not contaminate a latent electrostatic image bearing member such as a photoconductor drum is selectively used. Examples of such resins are urea resin, melamine resin, alkyd resin, modified alkyd resins such as phenol-modified alkyd resin and silicone-modified alkyd resin, acrylic resin, silicone resin, fluororesin, phenolic resin, polyamide resin, epoxy resin, polyester resin, maleic acid resin, and urethane resin. It is also disclosed that one or more resins selected from the group consisting of urea resin, melamine resin and acrylic resin are preferable resins from the veiwpoints of film formation properties and close contact properties. It is also stated that these resin components may contain a conducting material such as carbon black.
However, the development roller disclosed in the above prior art references has a shortcoming that the quantity of charge with which toner is charged by the development roller is low, so that when the charged toner contains many toner particles charged with an opposite polarity, such oppositely charged toner particles are deposited on the background of latent electrostatic images, and output images formed on a transfer sheet are such images that toner particles are deposited on the background of the images. This phenomenon is referred to as the fogging of background.
When the charge quantity of the toner is low, and its charging rise-up rate is low, the stability of image density is poor, for instance, with the formation of a difference in image density between a leading end portion and a rear end portion of an image transfer sheet, or the formation of ghost images.
Furthermore, under high temperature and high humidity conditions, the charge quantity of toner is apt to be lowered, and the fogging of the background of image is also apt to take place in output images on the image transfer sheet. Thus, the image density tends to become unstable.
Even when such problems do not occur at an initial stage in the use of the development roller, the above-mentioned problems may eventually occur during extended continuous use thereof, resulting in that the development roller has too poor a durability to be used in practice.
For example, it may occur that toner is deposited in the form of a film on the surface of the development roller as the number of copies made is increased in the course of making copies. This is caused by a mono-component toner for use in a mono-component contact development system being melted with a slight amount of heat applied thereto. More specifically, the surface layer of the development roller which bears thereon the mono-component toner is strongly brought into contact with other members, such as the photoconductor and a toner sealing member, so that the mono-component toner is melted with frictional heat generated by the strong contact of the surface layer of the development roller with the other members, and pressed hard to be fixed to the surface layer of the development roller with time. This fixing phenomenon is called "toner filming". The toner filming which takes place on the surface layer of the development roller lowers the charge quantity of toner, and causes much fogging of the background of output images, with the deposition of toner on the background of the images, making it difficult to obtain images with sufficient image density for use in practice.
The surface layer of the development roller is rotated in pressure contact with the photoconductor or a toner sealing member, so that there may occur a problem that the surface layer of the development roller is cracked during the use thereof for an extended period of time. Such cracking in the surface layer of the development roller appears in the form of an image defect in output images, so that it is required that the surface layer have such a flexibility that can sufficiently follow up a deformation of a depressed elastic layer.
Recently from the viewpoint of environmental problems, and also from the viewpoint of energy saving, the lowering of image fixing temperature has been studied. Under such circumstances, there is a tendency that the melting point of toner is lowered, so that there is less room for preventing the occurrence of the toner filming on the surface of the development roller.
Furthermore, recently the following are preferred: a development roller which is constructed in the form of a single unit so as to be detachable from an image formation apparatus; and a development unit, that is, a toner unit, in which the development roller is incorporated together with a charging member for applying charges to toner in a case for holding toner therein, and an image formation process unit, which is referred to as a process unit or a process cartridge, in which the development roller is integrated with a latent electrostatic image bearing member such as a drum-shaped photoconductor, which development unit and image formation process unit are constructed so as to be detachable from the image formation apparatus. Thus, by uniting a plurality of image formation members including the development roller for the image formation apparatus into a single unit, and incorporating the unit into the image formation apparatus, the maintenance of the unit including the image formation members can be simplified. In this case, even when one of the image formation members in the unit is found to be defective in the performance thereof, the unit can be replaced in its entirety with a perfect unit. Therefore, it is an extremely important target to be aimed at that the development roller used as a unit member in the above-mentioned unit has a sufficient high durability for continuous use for an extended period of time.
From the above-mentioned point of view, it is very important that the surface layer of the development roller is capable of applying charges to toner in a stable manner in continuous use for an extended period of time regardless of changes in the ambient conditions thereunder.